A wingless midge survives two winters frozen in its larval state.

When the term "extremophiles" gets thrown around, it's usually in reference to single-celled organisms that thrive in high salt or near-boiling water. But there are a few animals that also manage to make do in rather extreme conditions.

Perhaps the top example is a wingless midge that goes by Belgica antarctica. As its name implies, it's native to the frozen continent—in fact, it's the only insect that's native. (A few others have more recently introduced themselves from South America in recent years, and cockroaches undoubtedly ride in shipments to research bases.) Now, to try to help understand how anything can survive in such inhospitable conditions, researchers sequenced the genome of the midge and discovered it's gotten rid of a lot of the DNA that's frequently termed junk.

The researchers describe just how difficult the insect's living conditions are in detail: "The larvae, encased in ice for most of the year, require two years to complete their development and then pupate and emerge as adults at the beginning of their third austral summer. The [wingless] adults crawl over surfaces of rocks and other substrates, mate, lay eggs and die within 7–10 days after emergence."

The obvious hazard of this lifestyle—spending time frozen—is shrugged off by the midge. But the authors also note that Antarctica comes with a variety of additional environmental "onslaughts," as they put it. These include being dried out, getting exposed to both fresh and salt water, fried by UV light, and dealing with the rather intense chemical environments found where penguins and elephant seals make their homes.

In part, the insects survive this by controlling their gene expression such that a series of proteins that are normally activated during environmental shocks in other animals (termed heat shock proteins) are simply on all the time in the midge. But to get a better picture of the adaptations to Antarctica, some researchers took a single individual and sequenced its genome.

The most notable feature is that it's small, the smallest insect genome yet identified at under 100 million base pairs long. That's not an extreme outlier in the insect world (Drosophila, the fruit fly, is 130 million base pairs long), but it's still rather compact. But, despite the small size, the midge has roughly the same number of genes as other insects. As a result, the fraction of the genome that encodes a protein is nearly 20 percent; in humans, in contrast, that figure is about two or three percent.

How did this happen? Mostly by getting rid of lots of DNA that's generally labelled junk. These include repeated sequences and transposable elements, which can be thought of as DNA-level parasites that can move around within the genome. There are also sequences that interrupt genes and need to be spliced out of the RNA made from them called introns. They're still present in the midge, but they're generally shorter.

There are some changes in the gene content, however. The midge has more copies of genes that control development and regulate metabolism, which suggests that new copies of genes have been used to adjust to its odd developmental cycle and frequent environmental shocks. But one specific class of genes has been significantly reduced: odorant receptors, which allow animals to smell their environment. You could make a joke about this being an adaptation to being surrounded by penguin guano, but the authors suggest that this may be a result of the fact that the insects don't need to find a mate using pheromones.

In any case, the midge adds to a list of organisms that have very compact genomes. In all of these cases, the creatures have done so by getting rid of the sorts of DNA that are termed junk—DNA without an obvious function that isn't maintained as populations evolve. Since junk DNA can arise through accidental processes, its presence appears to be an accident and most organisms simply tolerate it because it doesn't cause them any consequences.

The neat thing about this is that it provides us with another example of where junk DNA gets eliminated. We saw that with a plant called the bladderwort, which has streamlined its DNA in response to a form of nutrient starvation. The midge's genome suggests that harsh environmental conditions might accomplish the same thing.